Communication systems that operate over limited spectral bandwidths must make highly efficient use of the scarce bandwidth resource to provide acceptable service to a large population of users. Examples of such communications systems that deal with high user demand and scarce bandwidth resources are wireless communications systems, such as cellular and personal communications systems.
Various techniques have been suggested for use in such systems to increase bandwidth-efficiency—the amount of information that can be effectively transmitted within a given spectral bandwidth. Many of these techniques involve reusing the same communication resources for multiple users while maintaining the identity of each user's signal. These techniques are generically referred to as multiple access techniques or protocols. Among these multiple access protocols are Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), Space Division Multiple Access (SDMA), and Frequency Division Multiple Access (FDMA). The technical foundations of these multiple access protocols are discussed, for example, in the recent book by Rappaport entitled “Wireless Communications Principles and Practice”, Prentice Hall, 1996.
The Time Division Multiple Access (TDMA) protocol involves the transmission of information from a multiplicity of users on one assigned frequency bandwidth by time division multiplexing the information from the various users. In this multiplexing scheme, particular time slots are devoted to specific users. Knowledge of the time slot during which any specific information is transmitted, permits the separation and reconstruction of each user's message at the receiving end of the communication channel.
The Code Division Multiple Access (CDMA) protocol involves the use of a unique code to distinguish each user's data signal from other users' data signals. Knowledge of the unique code with which any specific information is transmitted, permits the separation and reconstruction of each user's message at the receiving end of the communication channel. There are four types of CDMA protocols, classified by the specific technique that is used to spread the user's data over a wide portion of the frequency spectrum: direct sequence (or pseudo-noise), frequency hopping, time hopping, and hybrid systems. The technical foundations for CDMA protocols are discussed, for example, in the recent book by Prasad entitled “CDMA for Wireless Personal Communications”, Artech House, 1996.
The Direct Sequence CDMA (DS-CDMA) protocol involves the spreading of a user's data signal over a wide portion of the frequency spectrum by modulating the data signal with a unique code signal that is of higher bandwidth than the data signal. The frequency of the code signal is chosen to be much larger than the frequency of the data signal. The data signal is directly modulated by the code signal and the resulting encoded data signal modulates a single, wideband carrier that continuously covers a wide frequency range. After transmission of the DS-CDMA modulated carrier signal, the receiver uses a locally generated version of the user's unique code signal to demodulate the received signal and obtain a reconstructed data signal. The receiver is thus able to extract the user's data signal from a modulated carrier that bears many other users' data signals.
The Frequency Hopping Spread Spectrum (FHSS) protocol involves the use of a unique code to change the value of a narrowband carrier frequency for successive bursts of the user's data signal. The value of the carrier frequency varies in time over a wide range of the frequency spectrum in accordance with the unique code. CDMA protocols are closely related to spread spectrum technology and the term Spread Spectrum Multiple Access (SSMA) is also used for CDMA protocols such as DS-CDMA and FHSS that use a relatively wide frequency range over which to distribute a relatively narrowband data signal.
The Time Hopping CDMA (TH-CDMA) protocol involves the use of a single, narrow bandwidth, carrier frequency to send bursts of the user's data during intervals determined by the user's unique code.
Hybrid CDMA systems employ a combination of two or more CDMA protocols, such as direct sequence/frequency hopping (DS/FH), direct sequence/time hopping (DS/TH), frequency hopping/time hopping (FH/TH), and direct sequence/frequency hopping/time hopping (DS/FH/TH).
The CDMA protocols modulate each user's information with a different code unique to that user. Each user's information is separated and reconstructed at the receiving end of the communication channel by isolating that portion of the multiplexed signal that correlates with the user's code. In specific embodiments, orthogonal codes are used, permitting the complete separation of information associated with different codes, without cross-talk. If orthogonal codes are not employed, “code nulling” may be employed to limit interference due to correlation between various codes. This technique involves the judicious selection of codes that, though non-orthogonal, result in only minimal cross-talk.
The Space Division Multiple Access (SDMA) transmission protocol involves the formation of directed beams of energy, whose radiation patterns do not overlap spatially with each other, to communicate with users at different locations. Adaptive antenna arrays can be driven in phased patterns to simultaneously steer energy in the direction of selected receivers. With such a transmission technique, the other multiplexing schemes can be reused in each of the separately directed beams. For example, the same specific CDMA codes can be used in two different spatially separated beams. Accordingly, if the beams do not overlap each other, different users can be assigned the same code as long as they can be uniquely identified by a specific beam/code combination.
The SDMA receive protocol involves the use of multi-element adaptive antenna arrays to direct the receiving sensitivity of the array toward selected transmitting sources. Digital beamforming is used to process the signals received by the adaptive antenna array and to separate interference and noise from genuine signals received from any given direction. For a receiving station, received RF signals at each antenna element in the array are sampled and digitized. The digital baseband signals then represent the amplitudes and phases of the RF signals received at each antenna element in the array. Digital signal processing techniques are then applied to the digital stream from each antenna element in the array. The process of beamforming involves the application of weight values to the digital signals from each antenna element, thereby adjusting the numerical representation of their amplitudes and phases such that when added together, they form the desired beam—i.e., the desired directional receive sensitivity. The beam thus formed is a digital representation within the computer of the physical RF signals received by the antenna array from any given direction. The process of null steering at the transmitter is used to position the spatial direction of null regions in the pattern of the transmitted RF energy. The process of null steering at the receiver is a digital signal processing technique to control the effective direction of nulls in the receiver's gain or sensitivity. Both processes are intended to minimize inter-beam spatial interference. SDMA techniques using multi-element antenna arrays to form directed beams are disclosed in the context of mobile communications in Swales et. al., IEEE Trans. Veh. Technol. Vol. 39. No. 1 February 1990, and in U.S. Pat. No. 5,515,378. The technical foundations for SDMA protocols using adaptive antenna arrays are discussed, for example, in the recent book by Litva and Lo entitled “Digital Beamforming in Wireless Communications”, Artech House, 1996.
The Frequency Division Multiple Access (FDMA) protocol services a multiplicity of users over one frequency band by devoting particular frequency slots to specific users, i.e., by frequency division multiplexing the information associated with different users. Knowledge of the frequency slot in which any specific information resides permits reconstruction of each user's information at the receiving end of the communication channel.
Orthogonal Frequency Division Multiplexing (OFDM) addresses a problem that is faced, for example, when pulsed signals are transmitted in an FDMA format. In accordance with principles well known in the communication sciences, the limited time duration of such signals inherently broadens the bandwidth of the signal in frequency space. Accordingly, different frequency channels may significantly overlap, defeating the use of frequency as a user-identifying-parameter, the principle upon which FDMA is based. However, as discussed immediately below, pulsed information that is transmitted on specific frequencies can be separated, in accordance with OFDM principles, despite the fact that the frequency channels overlap due to the limited time duration of the signals.
OFDM requires a specific relationship between the data rate and the carrier frequencies. Specifically, the total signal frequency band is divided into N frequency sub-channels, each of which has the same data rate 1/T. These data streams are then multiplexed onto a multiplicity of carriers that are separated in frequency by 1/T. Multiplexing signals under these constraints results in each carrier having a frequency response that has zeroes at multiples of 1/T. Therefore, there is no interference between the various carrier channels, despite the fact that the channels overlap each other because of the broadening associated with the data rate. OFDM is disclosed, for example, by Chang in Bell Sys. Tech. Jour., Vol. 45, pp. 1775–1796, December 1966, and in U.S. Pat. No. 4,488,445.
Parallel Data Transmission is a technique related to FDMA. It is also referred to as Multitone Transmission (MT), Discrete Multitone Transmission (DMT) or Multi-Carrier Transmission (MCT). Parallel Data Transmission has significant calculational advantages over simple FDMA. In this technique, each user's information is divided and transmitted over different frequencies, or “tones”, rather than over a single frequency, as in standard FDMA. In an example of this technique, input data at NF bits per second are grouped into blocks of N bits at a data rate of F bits per second. N carriers or “tones” are then used to transmit these bits, each carrier transmitting F bits per second. The carriers can be spaced in accordance with the principles of OFDM.
A benefit of parallel data transmission derives from certain computational advantages associated with this transmission technique. Specifically, it can be shown that a parallel data signal is equivalent to the Fourier transform of the original serial data train and that the demodulation of the tones is equivalent to the inverse Fourier transform. This has led to the advantageous use of fast Fourier transform techniques (FFT) in implementing this technique, rather than the use of an expensive system of sinusoidal generators, modulators and coherent demodulators. See, for example, Weinstein and Ebert, IEEE Trans. on Comm. Tech., Vol. com-19, No. 5, October 1971, page 628.
Parallel data transmission can be used to service a multitude of users by dedicating specific tones to specific users. In this technique, specific information can be uniquely associated with any particular user by transmitting information only on that user's assigned set of frequencies or tone set. The use of multiple frequencies for one user permits the spreading of the signal over a wide, though discrete, portion of the frequency domain with the benefits familiar from spread spectrum communications. See U.S. Pat. No. 5,410,538 issued to Roche and Wyner.
Further multiplexing can be obtained by reusing the same set of frequencies or tone set for different users by modulating the tone set based on a user specific spreading code. Users assigned to the same tone set can then be distinguished by separating that portion of the multiplexed signals that correlate with their assigned code. See Yee, Linnartz, and Fettweis, “Multicarrier CDMA in indoor wireless radio networks,” Proc. PIMRC '93, Yokohama, Japan, pp. 109–113, September 1993.
Both the phase and the amplitude of the carrier can be varied to represent the signal in multitone transmission. Accordingly, multitone transmission can be implemented with M-ary digital modulation schemes. In an M-ary modulation scheme, two or more bits are grouped together to form symbols and one of the M possible signals is transmitted during each symbol period. Examples of M-ary digital modulation schemes include Phase Shift Keying (PSK), Frequency Shift Keying (FSK), and higher order Quadrature Amplitude Modulation (QAM). In QAM a signal is represented by the phase and amplitude of a carrier wave. In high order QAM, a multitude of points can be distinguished on a amplitude/phase plot. For example, in 64-ary QAM, 64 such points can be distinguished. Since six bits of zeros and ones can take on 64 different combinations, a six-bit sequence of data symbols can, for example, be modulated onto a carrier in 64-ary QAM by transmitting only one value set of phase and amplitude, out of the possible 64 such sets.
Suggestions have been made to combine some of the above temporal and spectral multiplexing techniques. For example, in U.S. Pat. No. 5,260,967, issued to Schilling, there is disclosed the combination of TDMA and CDMA. In U.S. Pat. No. 5,291,475, issued to Bruckert, and in U.S. Pat. No. 5,319,634 issued to Bartholomew, the combination of TDMA, FDMA, and CDMA is suggested.
Other suggestions have been made to combine various temporal and spectral multiple-access techniques with spatial multiple-access techniques. For example, in U.S. Pat. No. 5,515,378, filed Dec. 12, 1991, Roy suggests “separating multiple messages in the same frequency, code, or time channel using the fact that they are in different spatial channels.” Roy suggests specific application of his technique to mobile cellular communications using an “antenna array”. Similar suggestions were made by Swales et. al., in the IEEE Trans. Veh. Technol. Vol. 39. No. 1 February 1990, and by Davies et. al. in A.T.R., Vol. 22, No. 1, 1988 and in Telecom Australia, Rev. Activities, 1985/1986 pp. 41–43.
In U.S. Pat. No. 5,260,968, filed Jun. 23, 1992, Gardner and Schell suggest the use of communications channels that are “spectrally disjoint” in conjunction with “spatially separable” radiation patterns. The radiation patterns are determined by restoring “self coherence” properties of the signal using an adaptive antenna array. “[A]n adaptive antenna array at a base station is used in conjunction with signal processing through self coherence restoral to separate the temporally and spectrally overlapping signals of users that arrive from different specific locations.” See the Abstract of the Invention. In this patent, however, adaptive analysis and self coherence restoral is only used to determine the optimal beam pattern; “. . . conventional spectral filters . . . [are used] . . . to separate spatially inseparable filters.”
Winters suggests “adaptive array processing” in which “[t]he frequency domain data from a plurality of antennas are . . . combined for channel separation and conversion to the time domain for demodulation.” See U.S. Pat. No. 5,481,570, filed Oct. 20, 1993, Column 1 lines 66–67 and Column 2, lines 14–16.
Agee has shown that “the use of an M-element multiport antenna array at the base station of any communication network can increase the frequency reuse of the network by a factor of M and greatly broaden the range of input SINRs required for adequate demodulation. . . ” (“Wireless Personal Communications: Trends and Challenges”, Rappaport, Woerner and Reed, editors, Kluwer Academic Publishers, 1994, pp. 69–80, at page 69. See also, Proc. Virginia Tech. Third Symposium on Wireless Personal Communications, June 1993, pp. 15–1 to 15–12.) Agee asserts that in this aspect of his work “[s]patial diversity can be exploited for any networking approach and modulation format, by employing a multiport adaptive antenna array to separate the time-coincident subscriber signals prior to the demodulation operation [underlining added].” op. cit. page 72. In that same work, Agee separately demonstrates that the problem of receiving “signals over greatly disparate propagation ranges” . . . “can be overcome by exploiting the . . . spectral diversity inherent to the modulation format employed by typical communication networks.” op. cit. page 69. Considering CDMA networks, Agee shows that “the single-antenna received data signal . . . can be transformed to . . . a vector sequence. . . [that] . . . bears a strong resemblance to the signal generated by a narrowband antenna array receiving . . . spatially coherent signals in the presence of background interference.” op. cit. p. 76. The discussion is in terms of “CDMA networks employing an M-chip modulation-on-symbol (MOS) DSSS spreading format . . . ” op. cit. p. 69. (DSSS is the abbreviation for the direct sequence spectrum spreading or DS-CDMA protocol.)
Gardner and Schell, in U.S. Pat. No. 5,260,968, filed Jun. 23, 1992, also suggest “time division multiplexing of the signal from the base station and the users” . . . “[i]n order to use the same frequency for duplex communications. . . ” “[R]eception at the base station from all mobile units is temporally separated from transmission from the base station to all mobile units.” Column 5, lines 44ff. In a similar vein, in U.S. Pat. No. 4,383,332 there is disclosed a wireless multi-element adaptive antenna array SDMA system where all the required adaptive signal processing is performed at baseband at the base station through the use of “time division retransmission techniques.”
Fazel, “Narrow-Band Interference Rejection in Orthogonal Multi-Carrier Spread-Spectrum Communications”, Record, 1994 Third Annual International Conference on Universal Personal Communications, IEEE, 1994, pp. 46–50 describes a transmission scheme based on combined spread spectrum and OFDM. A plurality of subcarrier frequencies have components of the spreaded vector assigned to them to provide frequency-diversity at the receiver site. The scheme uses frequency domain analysis to estimate interference, which is used for weighting each received subcarrier before despreading. This results in switching off those subcarriers containing the interference.
Other disclosures of interest in this area include:
N. Yee, Jean-Paul M. G. Linnarta, G. Fettweis, “Multi-Carrier CDMA in Indoor Wireless Radio Networks”, IEICE Transactions on Communications, Vol. E77-B, No. 7 pp. 900–904, July 1994;
L. Vandendorpe, “Multitone Spread Spectrum Multiple Access Communications System in a Multipath Rician Fading Channel”, IEEE Transactions on Vehicular Technology, Vol. 44 No. 2, pp. 327–337, May 1995;
L. Vandendorpe, “Multitone Direct Sequence CDMA System in an Indoor Wireless Environment”, IEEE First Symposium on Communications and Vehicular Technology, Benelux Delft Netherlands, pp. 4.1–1 to 4.1–8, Oct. 27–28, 1993; and
K. Fazel, “Performance of CDMA/OFDM for Mobile Communication System”, 2nd IEEE International Conference on Universal Personal Communications, Otawa, Ontario, pp. 975–979, Oct. 12–15, 1993.
The following references describe various methods to combine adaptive beamforming with processing the spreading codes in CDMA:
G. Tsoulos, et al. “Adaptive Antennas for third generation DS-CDMA cellular systems”, Proc. IEEE VTC'95, pp. 45–49, August 1995.
Y. Wang et al., “Adaptive antenna arrays for cellular CDMA communication systems”, Proc. IEEE Intl. Conf. Acoustics, Speech and Signal Processing, Detroit, pp. 1725–1728, 1995.
B. Quach, et al, “Hopfield network approach to beamforming in spread spectrum communications”, IEEE Proc. Seventh SP Workshop on Statistical Signal and Array Processing, pp. 409–412, June 1994.
A. Sandhu, et al. “A Hopfield neurobeamformer for spread spectrum communications”, Sixth IEEE Int. Symposium on Personal, Indoor and Mobile Radio Communications, September 1995 (no page given)
A. F. Naguib, et al. “Performance of CDMA cellular networks with base-station antenna arrays”, in C. G. Gunther, ed. “Mobile Communications—Advanced systems and components”, Springer-Verlag, pp. 87–100, March 1994.
V. Ghazi-Moghadam, et al, “Interference cancellation using adaptive antennas”, Sixth IEEE Int. Symposium on Personal, Indoor and Mobile Radio Communications, pages 936–939, September 1995.
H. Iwai, et al. “An investigation of space-path hybrid diversity scheme for base station reception in CDMA mobile radio”, IEEE J Sel. Areas, Comm., vol. SAC-12, pp. 962–969, June 1994.
R. Kohno, et al. “A spatially and temporally optimal multi-user receiver using an array antenna for DS/CDMA”, Sixth IEEE Int. Symposium on Personal, Indoor and Mobile Radio Communications, Toronto, pages 950–954, September 1995.
Despite these suggestions to combine certain of the multiple access protocols to improve bandwidth efficiency, there has been little success in implementing such combinations. One reason for this lack of success is that it becomes more difficult to calculate optimum operating parameters as more protocols are combined. The networks implementing combined multiple access protocols become more complex and expensive. Accordingly, the implementation of high-bandwidth efficiency communications using a combination of multiple access protocols continues to be a challenge.